An old farm hand put me wise once... Cover the roof with sacks(Hessian) he said and hose it down regularly. I also took store regards the chimneys like used in piggeries and Battery hens etc.
The higher the chimney the better the draw of course. I did once. Later I done one massive one. The whole roof was over a mesh roof. No matter how much, or little wind / air, it cooled the inside out and kept it fresh smelling.